Passengers who board a District taxi at National Airport and ask the cabbie to drive them into Washington may be taken for more than a ride across the Potomac. Chances are they also will be overcharged.

During one recent week, two Washington Post reporters made a total of 10 trips at different times of the day from National to the Shoreham Hotel at Connecticut Avenue and Calvert Street NW. On every trip, the reporters were overcharged. Overcharges ranged from $1.10 to $3.50 for the $7.40 ride.

The fare between the airport and the Shoreham via Rock Creek Parkway is $6.90 plus a 50-cent airport dispatching fee, according to Maurice J. Harmon, rate supervisor at the Washington Metropolitan Area Transit Commission (WMATC), which sets fares for taxi trips across state lines into the District. Washington cabs do not have meters; instead, cabbies compute fares by mileage. Harmon said some flexibility in computing the seven-mile trip is possible because of improperly calibrated odometers, but in any event, the fare should be no higher than $7.85.

A Yellow Cab charged one reporter $8.50 on Nov. 9, the lowest fare of the 10 trips. The highest fare was $10.90, charged Nov. 10 by an independent cabdriver. Most of the other fares were about $9. Another driver, who had charged $9 for the trip, voluntarily gave a reporter a business receipt for $11 and told him to pocket the difference.

Cabdrivers are required to charge incremental fees based on fractions of miles traveled, but only two of the 10 fares appeared to be based on the fare structure. And in both of those cases, the charges were for more than the distance traveled. Also, none of the 10 cabdrivers had their licenses posted in the cab as required, and one cabbie had to ask another cabdriver for directions to the Shoreham Hotel.

Post reporters are not the only people who have been overcharged. "I cannot remember when last I took a cab from or to National Airport that the cabdriver did not, deliberately and knowingly, overcharge me," said Timothy A. Hanan, a New York businessman who flies to National Airport several times a month and rides taxis into the city.

"It's flagrant," Hanan said yesterday during a telephone interview. "I was in Washington Monday and a cabbie charged me $7.50 for a $5.50 cab ride." Hanan has filed so many complaints with the WMATC that officials there know him on a first-name basis. Recently, he has received four refunds through WMATC staff, ranging from $1.60 to $2.30.

It is difficult for passengers -- especially those unfamiliar with Washington's unmetered cabs -- to figure out the correct fares, Hanan said. "The only way to make sure you aren't getting ripped-off is to lean over the seat and look at the odometer at the beginning of the trip and at the end," he said.

The WMATC has received 366 written complaints about overcharging this year. "Those are just the folks who write us," Harmon said. "A lot more call us up and then just hang up when they realize they have to write us a letter before we can do anything."

Most of the time, a cabbie offers to refund the overcharge once the WMATC contacts him, Harmon said. If the cabbie refuses, the WMATC sends a complaint to the D.C. Hacker's License Appeal Board, which has the power to revoke a cabbie's license.

Even then, the cabbie has a good chance of going unpunished, according to Harold Foster, a spokesman for the board. "The complaining passenger has to be willing to attend a hearing and most of the people who complain are professionals who just don't have time to spend an hour or so at a hearing over a few dollars."

Cabbies who overcharge, Foster said, "know exactly what they are doing. They are playing the percentages, and even if they are caught 1 time out of 10, they still get away with it nine times." In the past, the board has averaged 250 complaints each year about overcharging by D.C. hackers on both local and interstate trips. But this year alone, it has received 600 complaints. Foster says the increase follows the city's use of public service spots on radio and television to encourage passengers to complain and because of an increase in overcharging by some of the city's 9,213 licensed cabbies.

Overcharging has become such a problem that earlier this year, WMATC held its own revocation hearing -- the first in its 20-year history -- and forbade a cabbie from making trips across the District line. That cabbie had charged $74.20 for a $21 trip from National Airport to Bowie.

A comment by a spokesman for the Liberty Cab Co. was typical of responses from companies when told their drivers had overcharged the Post reporters. "We can't do anything about it," said the spokesman, who refused to give his name. "We simply rent out cabs. One hundred bucks a week for a '76 Plymouth, you keep whatever you make . . . . When we get complaints we just tell them to call the hackers' board."

Most of Liberty's 800 cabs are rented to foreigners living in Washington, and many of the drivers pretend to be ignorant about the fare structure as a pretext for overcharging, the spokesman acknowledged.

Yellow Cab Co. said it fires drivers who get more than three complaints per year. "It's just a handful of bad drivers that are giving everyone else a bad name," said company general manager Frank Greshman.

Hanan doesn't believe that.

He claims he has been overcharged nearly every time he has ridden a taxi to or from National Airport. But Monday, something strange happened, Hanan said. A cabbie charged him the correct fare for a ride to the airport.

"I was so shocked I couldn't believe it," Hanan said. "The fare was only $5.50, but I gave him a $3 tip because I'd finally found an honest cabdriver in Washington." CAPTION: Picture 1, Taxis, lined up for fares at National Airport, charge different rates for trips to the same destinations. By Tom Allen -- The Washington Post; Picture 2, Receipts for rides in four cabs from National Airport to the same DC. hotel range from $8.50 to $11. The Washington Post